The present invention relates to cryptography. Specifically, the invention relates to encryption systems which mix an input signal with a signal corresponding to a pseudo random number, in order to encrypt the input signal.
There is considerable need for high speed encrypting of information. High speed encrypters are utilized, for example, in numerous aerospace applications such as low probability of intercept (LPI) communications, satellite communications (COMSAT), multi-level secure local area networks (MLS-LAN) and agile radars, as well as in other applications. The levels of encryption required vary in each case For example, LPI applications may require only a few hours of security whereas MLS-LAN information may be required to be secure for several years. In all applications, high speed, that is greater than 100 MHz, is required.
Encryption involves combining an input data, or plaintext, stream with a pseudo random number sequence (PRS) data stream at a transmitter end, and decryption involves essentially the reverse procedure at a receiver end. This reversal process requires the receiver to generate the same PRS that was used to encrypt the input data at the transmitter end. A "key" is usually passed from the transmitter end to the receiver end which informs the receiver end of the PRS being employed.
The length of the PRS is defined as the number of characters in the PRS before the PRS repeats. The difficulty of breaking a cipher is related to the length of the PRS. The longer the PRS, the more difficult the cipher is to break. Thus, a good encrypter must generate a long PRS without excessive hardware. Cipher Systems The Protection of Communications, by H. Beker and F. Piper (John Wiley & Sons, Inc., New York 1982) pages (59-71, provides a general background discussion of cryptographic devices, including the M-209 Converter discussed below and is incorporated herein by reference.
An example of a prior art method used to generate long PRS's will be discussed with reference to FIG. 1. The electronic method illustrated in FIG. 1 is analogous to the mechanical method employed in the widely used M-209 Converter and its derivatives. In FIG. 1, a set of six short length binary PRS's 110, 120, 130, 140, 150, and 160 are stepped past a demultiplexer and reading point 50 to form a six bit binary number. The six short length binary PRS's 110, 120, 130, 140, 150, and 160 have lengths of 17, 19, 21, 23, 25, and 26 bits, respectively. These lengths are carefully chosen so that no two of them have a common factor, that is, these lengths are mutually prime numbers. As the six short length binary PRS's 110, 120, 130, 140, 150, and 160 are stepped past the demultiplexer and reading point 50 in time, a longer PRS is generated. If the six short length binary PRS's 110, 120, 130, 140, 150, and 160 are internally aperiodic, the length of the longer PRS is the product of the lengths of the individual short length binary PRS's or 101, 405, 850. The six bit binary number is used as an address to specify a 5 bit word W, having digits w.sub.1 . . . w.sub.5, stored in a read only memory unit 60. This 5-bit word W is applied to a number T representing a single alphabetic character of a plaintext message in an adder 70 using addition module 26 to produce encrypted message S. It should be noted that in the M-209. Converter itself, the reduction analogous to the reduction performed by the read only memory unit 60 is done mechanically by lugs and a lug cage.
The prior art encrypters, such as the one described, do not provide an acceptable degree of encryption at high speed. This lack of speed and lack of acceptable encryption is in part due to the nature of the binary arithmetic required by these prior art encrypters.